1. FIELD OF THE INVENTION
The present invention relates to an apparatus which uses laser beams to reproduce signals recorded on optical disk, and more particularly to an optical disk reproducing apparatus of the type in which the pitch of the tracks on a optical disk is reduced to realize a high-density and high-transfer-rate recording and reproduction and which is designed to perform tracking control with greater accuracy.
2. DESCRIPTION OF THE PRIOR ART
Various optical disk apparatus of the type employing a laser light to record and reproduce various information from a disk have recently been proposed. With a view to increasing the recording density and transfer rate, we have proposed a method for recording and reproducing a signal by utilizing an oblique surface of helical or circular grooves having a V-shaped section with respect to an imaginary sectional plane a radial direction of disk (U.S. patent application Ser. No. 525,411 filed on Aug. 22, 1983).
Referring now to FIG. 2 of the accompanying drawings, there is illustrated a perspective view showing the radial section of an optical disk formed with V-grooves. In FIG. 2, numeral 41 designates the substrate of the disk having V-grooves on its surface having oblique surfaces designated by A, B, C, D ---. A recording thin film 42 such as of TeOx (x.apprxeq.1.1) is formed on the surfaces of the V-grooves. As shown in FIG. 3, laser spots 1 and 2 are respectively projected to the center of the adjacent slopes, e.g., A and B or C and D in FIG. 2. By driving the laser spots independently of each other, it is possible to record independent signals on the two slopes by changing reflectivity of the small areas 3 on the track by producing reaction by the laser beam spot.
Next, the method of reproducing the signals recorded in this way will be described briefly. As in the case of FIG. 3, the laser spots 1 and 2 are respectively projected to the slopes C and D. Signals recorded on the two tracks are simultaneously reproduced, and such signals are usable, for instance, to obtain a transfer rate as high as twice of the single track optical disk system. As disclosed in the specification of the above mentioned U.S. patent application, if the V-grooves are formed into an optimum shape, crosstalk from a neighboring oblique surface is sufficiently suppressed through receiving mainly .+-. first-order diffraction lights among reflection lights from the disk, and each signal of the respective tracks can be individually reproduced.
The tracking method used with the above-described recording and reproducing method will now be described.
The so-called push-pull method is used for the tracking control. As shown in (a) of FIG. 4, a tracking control laser spot 10 is arranged in such a manner that the laser spot 10 is positioned at the center of the V groove when the signal recording and reproducing laser spots 1 and 2 are positioned centrally on the tracks C and D, respectively. Shown in FIG. 4(b) is a tracking control circuit used for the tracking control. In FIG. 4(b), numerals 11 and 12 designate bisplit photosensors, 13 and 14 preamplifiers, 15 a subtractor, 17 an objective driver circuit, and 18 an objective lens.
In accordance with the push-pull method, the photosensors 11 and 12 are arranged in such a manner that their dividing line 16 coincides with the center of the V-groove on the far-field images of the reflected beams of the controlling laser spot 10 from the optical disk. Where the controlling laser spot 10 is positioned at the center of the V groove as shown in FIG. 4(a), the far-field images of the reflected beams from the optical disk are symmetrical with resect to the center of the V groove and the outputs of the photosensors 11 and 12 are equal. These photosensor outputs are respectively amplified by the preamplifiers 13 and 14, thereby producing tracking detection signals t.sub.1 and t.sub.2. The subtractor 15 obtains the difference between the tracking detection signals t.sub.1 and t.sub.2 to produce a tracking error signal t.sub.e. The tracking error signal t.sub.e is given by the following equation. EQU t.sub.e =t.sub.1 -t.sub.2 ( 1)
In the case shown in FIG. 4(a), the tracking error signal t.sub.e is zero. Then, where the position of the controlling laser spot 10 is shifted to the track C side, the intensity distributions of the far-field images of the reflected beams are unsymmetrical with respect to the center of the V groove so that as for example, the quantity of light incident to the photosensor 11 is increased. In this case, the value of the tracking error signal t.sub.e becomes positive. The tracking error signal t.sub.e is applied, for example, to the objective driver circuit 17 in the optical head section so that the objective lens 18 is actuated to move the laser spot 10 toward the track D. On the contrary, where the position of the controlling laser spot 10 is shifted toward the track D side, the value of the tracking error signal t.sub.e becomes negative and the objective driver circuit 17 actuates the objective lens 18 to move the laser spot 10 toward the track C.
By thus using the push-pull method to effect the tracking control such that the controlling laser spot 10 is always positioned at the center of the V groove, it is possible to respectively position the signal recording and reproducing laser spots 1 and 2 at the center of the respective slopes.
Recently, it has been practiced to record and reproduce digital signals representing various information from optical disks. Generally, such digital signals are modulated by using a modulation code suited to the recording and reproducing characteristics of the optical disk and the signals are recorded by changing an optical characteristic, e.g., reflectance of the recording medium. With the modulation code for digital signals, generally a low-frequency component is included in the frequency components of the signals after the modulation. For example, the modulated signal by the NRZ modulation code includes a d.c. component, too. Also, even in the case of a modulation system involving no d.c. component, e.g., the FM modulation code, a low-frequency component such as included in the frequency band of the tracking signals is involved. Thus, when reproducing the optical disk on which such digital signals have been recorded, crosstalk is caused in the tracking signals by the reproduced signals. The generation of the crosstalk component from the reproduced signals represents the fact that the average reflectances of the reproduced tracks have changed. Therefore, particularly in such cases where the tracking control is effected on the ridge of the adjacent tracks by the push-pull system as in the case of the conventional system, the magnitudes of crosstalk in the respective tracks are not the same and thus the tracking error is increased.
For instance, where the average reflectance of the track C is greater than that of the track D in FIG. 4(a), even if the controlling laser spot 10 is positioned at the center of the V groove, the reflected light from the track C is increased so that the incident light quantity to the photosensor 11 is increased as compared with the photosensor 12 and the tracking error signal t.sub.e has a positive value. This is the same with the case in which the position of the controlling laser spot 10 is shifted toward the track C and thus the objective lens 18 is actuated to move the position of the laser spot 10 toward the track D until the tracking error signal t.sub.e is reduced to zero. Thus, where there is the difference in reflectance between the adjacent tracks of the V groove, the position of the controlling laser spot is varied thus making it difficult to effect the tracking control accurately.